re-corede deck, new top skin

Discussion in 'Fiberglass and Composite Boat Building' started by dahlke, Aug 2, 2017.

  1. dahlke
    Joined: Aug 2017
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    Location: Denmark

    dahlke Junior Member

    Hi :)

    I'm about half way through in replacing all the old partly delaminated and wet foam aboard a 38ft flush decked sailboat. Within the next 4 weeks, I should be done putting down the new core and be ready to start laying up the new top skin. I'm using divinycell h100 and west's 105 epoxy (+ 406 Silica).

    I've got 60 kg of 450g biaxial (orientated +/- 45) fabric. I don't want to reuse any of the old top skin (around 6 mm thick all CSM).

    Here come the questions:
    1) How thick of a top skin should I aim for using biaxial and epoxy (over the old polyester CSM)? Should I just go for 6mm again?
    2) Is there another fabric that would be more suited for the purpose than the 450g biaxial I've got?

    Any advice would be much appreciated :)

    This is my first post here so I'm not sure about how to best attach photos:

    https://goo.gl/photos/MgwTjBktRHMAkaAb7

    IMG_1158.JPG
     
  2. jorgepease
    Joined: Feb 2012
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    Location: Florida

    jorgepease Senior Member

    .25 inch of glass ??? that sounds really thick. Did you glass the bottom of the core? A major advantage of foam core is you don't need heavy skins to get good stiffness but you do need to glass both sides.
     
  3. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    Not only to glass both sides but to put, if possible, the same combination of layers on both sides. I agree, 6mm of glass would probably be too much but, of course, we do not know the loads that will support the deck.
     
  4. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    Frequently the top side of the core may be thicker than needed because you need to try and prevent impact damage, but with epoxy you may be able to get by with about half what was there.

    The bottom is going to be reused, so whatever it was it lasted all these years and held up, so it should be fine in the future.

    One other thing, I'd turn the boat over, it's not easy to do it upside down.
     
    Last edited: Aug 3, 2017
  5. dahlke
    Joined: Aug 2017
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    Location: Denmark

    dahlke Junior Member

    Thanks for your reply :)

    Hehe, yeah I'm sorry about the orientation of that picture. I don't know what went wrong there :)

    You are of course correct in assuming that I've reused the bottom skin. The foam is adhered to that using thickened epoxy (Silica).

    I guess I could always try laying up half the thickness of the old deck and then just add more if it turns out to be soft? :) Yay for sanding ;)
     
  6. dahlke
    Joined: Aug 2017
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    dahlke Junior Member

    The bottom of the core is adhered to the old skin using thickened epoxy :)
     
  7. dahlke
    Joined: Aug 2017
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    Location: Denmark

    dahlke Junior Member

    The (old) bottom skin that the new core is adhered to is a lot thinner than the top. I'd say closer to 2-3 mm. There is no harm in having the top skin be thicker than the bottom right? well other than maybe weight and cost :)
     
  8. heikki
    Joined: Jun 2015
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    Location: Finland

    heikki Junior Member

    What was the original top skin made of? Any oriented or just chopped fibers? Top skin is thicker just for possible impacts. If the new foam is stronger than the old then skin could be thicker too. With biax resin content will be lower than with just CSM and density higher. So 6 mm of that would be heavier than original and also a lot stronger. Try around 4 mm or even less unless someone has a suggestion of laminate schedule which gives lower density at the middle of the skin. How thick laminate do you think you can lay with one go? Do you plan to laminate smaller sections or the whole deck at once? Do you dare to try infuse the deck? If hand laying smaller sections first might make sense. Hand laying 6 mm of biax at one go sounds like a big job so there might be a chance to see how the deck feels when top skin is thinner than the original. Using peelply at top of the wet laminate is a good idea too. It keeps resin of running down and leaving voids in the laminate as well as prevents amine blush. Also on top of the final layer too. Will there be teak on the new deck?
    Here is a deck being laminated on top of foam core. Does not look if there was 6 mm of thickness on it:

    Fram's Blog https://framsblog.wordpress.com/page/8/

    Some off topic here:
    I have been following your YouTube channel. Ty for the entertainment. One question about the new core. At some stage of the build those cuts between foam cubes should be filled with resin in order to glue sides of cubes to each other. Do you fill the cuts while laying the core or is it possible to do after the core has been glued to bottom skin?
    Vacuum will not pull air out of thick laminate if some air has been left in while hand laminating it but only expands it so one has to do the job right even if vacuum is used to consolidate the laminate. The patch you made on the last video is anyhow stronger than the rest of the boat.
     

  9. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    From the point of view of structural strength, the laminates should be symmetrical with respect to the neutral axis of the deck panel. If you, for whatever reason, need to add some more layer on the outside, that can not be a problem but it is not too unbalanced the distribution of tensions in the thickness.
    Anyway, if you find it very difficult, forget all this. In normal small boats, the efforts on the deck are not great, unless there are some concentrated loads in some areas of the deck and that can be solved with local reinforcements.
     
    Last edited: Aug 3, 2017
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